The disclosed invention relates to memory devices and more particularly to two terminal bistable memory cells which comprise at least two high-spin polarization magnetic material junctions, said junctions being separated from one another by insulator, said insulator typically containing trap sites; said two terminal bistable memory cell demonstrating two stable, low A.C. or D.C. current readable, hysteretic resistance states which are set by the flow of a relatively higher plus or minus D.C. polarity current therethrough. Preferred cells each comprise at least one sequence of: CrO2/Cr2O3/CrO2.
State of the art electronic devices include, for instance, three terminal Complementary Metal Oxide Semiconductor (CMOS) devices which provide fan-in and fan-out capability, and can perform memory and logic functions. Where only memory is required, however, two terminal electrical devices are applicable. Two terminal electrical memory systems are functionally similar to magnetic dipoles which are set to one of two stable states by application of magnetic fields, but they are set by application of voltage/current and are monitored by applying a current/voltage and reading a voltage/current. Two terminal memory devices include Giant magnetoresistive devices as well as tunnel junction based magnetics.
Two terminal devices can operate based on a tunnel magnetoresistance effect and can be comprise a sequence of:
ferromagnetic/insulator/ferromagnetic;
materials. The resistance across said sequence is determined by the relative magnetic alignment of two ferromagnetic layers. The effect is based in the ability of ferromagnetics to polarize spin in electric currents where Polarization (P) is given by:
P=(n+xe2x88x92nxe2x88x92)/(n++nxe2x88x92);
where n+ and nxe2x88x92 are the number of conduction electrons with their spin parallel and anti-parallel, respectively. It is noted that optimum magneto-electronic properties correspond to a Polarization of 100%. One important candidate for 100% spin polarization is Chromium Oxide. Articles which show that CrO2 demonstrates significant magneto-resistance effects are:
xe2x80x9cEnhanced Intergrain Tunneling Magnetoresistance in Half-Metallic CrO2 Filmsxe2x80x9d, Hwang et al., Science 278, (1998);
xe2x80x9cMagnetoresistance of Chromium Dioxide Powder Compactsxe2x80x9d, Coey et al., Phys. Rev. Lett., 80, (1998);
xe2x80x9cExtrinsic Giant Magnetoresistance In Chromium (IV) Oxide, CrO2xe2x80x9d, Manoahran, Appl. Phys. Lett., 72 (1998).
Additional known references which are relevant to non-volatile memory devices which utilize stray magnetic fields generated by currents to switch resistance states include:
xe2x80x9cDouble Tunnel Junctions for Magnetic Random Access Memory Devicesxe2x80x9d, Iomata et al., J. Appl. Phys., Vol. 87, No. 9, (May 2000), which describes fabrication of double tunnel junctions which comprise a central ferromagnetic layer prepared by alternate sputter deposition of Co80Pt20 and Al2O3. Where said ferromagnetic layer has top and bottom electrodes made of FeCo applied thereto, room temperature Tunnel Magnetoresistance of 20% was achieved.
xe2x80x9cExchange-based Magnetic Tunnel Junctions and Application to Nonvolatile Magnetic Random Access Memoryxe2x80x9d, Parkin et al., J. Appl. Phys., Vol. 85, No. 8, (April 1999), describes tunnel junctions comprising two ferromagnetic layers separated by an insulating tunnel barrier. Switching between magnetoresistance states is shown as achieved by application of a magnetic field.
xe2x80x9cUltrahigh Density Vertical Magnetoresistive Random Access Memoryxe2x80x9d, Jian-Gang Zhu et al., J. Appl. Phys., Vol. 87, No. 9, (May 2000), mentions a ring shaped vertical magnetoresistive random access memory comprised of magnetic tunneling junctions.
xe2x80x9cSpin Dependent Tunneling Devices Fabricated for Magnetic Random Access Memory Applications Using Latching Modexe2x80x9d, Wang et al., J. App. Phys., Vol. 87, No. 9 (May 2000); describes Spin Dependent Tunneling Junctions comprising: NiFeCo/Al2O3/CoFe/IrMn formed by rf diode sputtering.
Continuing, it is generally accepted that spin polarized current density larger than about 107 A/cm2 is necessary to produce sufficient torque on a magnetic nanoparticle and change its orientation, thus that several Milliamps are required to flip regions in system fabricated by electron-lithography of a typical size of 100xc3x97100 nm area. It is also noted that asymetric results occur when positive and negative currents are applied. Articles which provide insight to non-volatile memory devices which operate based on spin transfer from electrons to set hysteretic resistance states are:
xe2x80x9cMagnetization Precession by Hot Spin Injectionxe2x80x9d, Weber et al., Science, 291, (2001) discloses experimental results which demonstrate that electron spins experience a torque when going through a ferromagnetic material. Following Newton""s Third Law the electrons produce an opposite effect on the magnetic material and can modify its magnetic orientation;
xe2x80x9cA New Twist for Magneticsxe2x80x9d, Ralph, Science, Vol. 291, (February 2001), which describes that electric currents can manipulate magnets not only by the well known effect of the translation motion of electrons, say through a wire etc., but that the spin of electrons can be beneficially used as well. This article describes that electrons first passed through a spin filter so that a flow thereof is populated predominately by electrons of the same spin, can specifically affect magnetic states on a less than 1 micron dimension scale, whereas magnetic field effects which are based on stray field effects decay slowly with distance, thereby limiting packing density of dipoles which can be specifically controlled thereby, without affecting nearby dipoles. The use of electron spin then makes possible smaller memory cells, and for devices with dimensions of less than 100 nm, electron spin becomes the dominate effect;
xe2x80x9cExcitation of Spin Waves by an Electric Currentxe2x80x9d, Slonczewski, J. Magn. Magn. Matter, 195, (1999);
xe2x80x9cEmission of Spin Waves by a Magnetic Multilayer Traversed by a Currentxe2x80x9d, Berger, Phys. Rev. B 54, (1996);
xe2x80x9cExcitation of a Magnetic Multilayer by an Electric Currentxe2x80x9d, Tsoi et al., Phys. Rev. Lett 80, (1998);
xe2x80x9cCurrent-Induced Magnetization Reversal in Magnetic Nanowiresxe2x80x9d, Wegrowe et al., Europhys. Lett. 45, (1999);
xe2x80x9cCurrent-Induced Switching of Domains in Magnetic Multilayer Devicesxe2x80x9d, Myers et al., Science 285, (1999);
xe2x80x9cSpin-Polarized Current Switching of a Co Thin Film Nanomagnetxe2x80x9d, Albert et al., Appl. Phys. Lett. 77, (2000);
xe2x80x9cSpin-Polarized Current Induced Switching in Co/Cu/Co Pillarsxe2x80x9d, Grollier et al., Appl. Phys. Lett. 78, (2001).
xe2x80x9cCurrent-Driven Switching of Magnetic Layersxe2x80x9d, Heide et al., Phys. Rev. B, Vol. 63, (2001);
xe2x80x9cCurrent-Driven Magnetic Switching in Manganite Trilayer Junctionsxe2x80x9d, Sun, J. of Magnetism and Magnetic Materials, 202 (1999);
xe2x80x9cCurrent Hysteresis Due to Changes in Magnetization of Magnetic Tunnel Junctions by Spin-Polarization Currentxe2x80x9d, Baranov, Europhys. Lett. 53 (5) (2001).
A great many papers report research into materials which provide bi-stable memories. The following are representative:
xe2x80x9cCurrent-Driven Insulator-Conductor Transition and Nonvolatile Memory in Chromium-Doped SrTiO3 Single Crystalsxe2x80x9d, Watanabe et al., J. App. Phys., Vol 78, No. 23, (June 2001), which describes non-volatile memory comprised of Chromium doped SrTiO3 single crystals in which D.C. current flow induced reversible insulator-conduction transition of up to five orders of magnitude, and
xe2x80x9cReproducible Switching Effect in Thin Oxide Films for Memory Applicationsxe2x80x9d, Beck et al., Appl. Phys. Lett. 77 (2000).
With the present invention in mind, a Search of Patents provided:
U.S. Pat. No. 6,069,820 to Inomata et al. describes a Spin Dependent Conduction Device. This Patent describes a sequence of:
Tunnel Junctionxe2x80x94Ferromagnetic Layerxe2x80x94Tunnel Junction
wherein a ferromagnetic material is sandwiched between tunneling junctions.
U.S. Pat. No. 5,841,692 to Gallagher et al. describes a magnetic tunneling junction device with antiferromagnetically coupled pinned layer.
U.S. Pat. No. 5,650,958 to Gallagher et al. describes a magnetic tunnel junction with controlled magnetic response.
U.S. Pat. No. 5,966,012 to Parkin describes a Magnetic tunnel junction device with improved fixed and free ferromagnetic layers.
U.S. Pat. No. 5,089,991 to Matthews describes a non-volatile memory cell which operates based on the Hall effect.
U.S. Pat. No. 6,166,948 to Parkin et al. describes a magnetic memory array with magnetic memory tunnel junctions.
U.S. Pat. No. 6,226,160 to Gallagher et al. describes a small area magnetic tunnel junction devices with low resistance and high magnetoresistance.
U.S. Pat. No. 6,215,695 to Ikeda describes a magnetoresistance element for application in memory.
U.S. Pat. No. 6,233,172 to Chen et al. describes a magnetic element with dual magnetic states.
U.S. Pat. No. 5,856,897 to Mauri describes a self-biased dual spin valve sensor.
U.S. Pat. No. 5,953,248 to Chen et al. describes a low switching field magnetic tunneling junction for use in high density arrays.
U.S. Pat. No. 6,215,696 to Tsuge describes a ferromagnetic tunnel junction device and method for its forming.
U.S. Pat. No. 6,178,112 to Bessho et al. describes a ferromagnetic material split by a spacer which comprises a magnetic material and a semiconductor material, the magnetic state of which can be set by application of a stimulus, such as a current flow.
Even in view of the cited references, there remains need for improved non-volatile two-terminal bistable memory systems, and methods of sensing states set therein.
The disclosed invention system can broadly be described as comprising a functional combination of:
a two-terminal static memory cell comprised of at least two junctions between magnetic materials, which junctions demonstrate electron spin preference tunneling; said memory cell being characterized in that application of a positive or negative D.C. voltage across the two terminals thereof causes preferred spin electron flow therethrough, thereby resulting in the setting of a monitorable resistance state therein, the utility being that at least two resistance states can be set by application of different polarity voltages; and
a means for sequentially applying D.C. state setting, and D.C. or A.C. sensing voltages/currents, said means for applying D.C. or A.C. sensing voltages/currents including means for monitoring D.C. or A.C. current/voltage.
Said disclosed invention two terminal memory cell is formed from ferro-magnetic and antiferro-magnetic materials, with the preferred ferro-magnetic and antiferro-magnetic materials being CrO2 and an insulator Cr2O3, respectively, which CrO2 and Cr2O3 can both be formed by oxidization of chromium to form Cr/CrO2(Cr2O3), or by vacuum deposition, (eg. sputtering), etc. thereby providing CrO2/Cr2O3/CrO2 sequences.
Present invention two-terminal static memory cells enable setting one or another stable state therein by the application of a D.C. voltage/current level of positive or negative polarity across said two terminals thereof. The stable state set is readable by application of a smaller A.C. or D.C. voltage/current in combination with monitoring current/voltage level caused thereby. Note specifically that the disclosed invention can be monitored by D.C. or A.C. a current flow magnitude response to a small applied D.C. or A.C. voltage, or by monitoring a D.C. or A.C. voltage response to an applied small D.C. or A.C. current.
A method of the disclosed invention involves providing a disclosed invention static memory cell as described above, then setting it into one or another stable state by the application of a D.C. voltage level in a range of D.C. voltages, and then monitoring or retrieving said set stable state by application of an A.C. or D.C. voltage in combination with monitoring current flow level caused thereby. Of course said method can include resetting the stable state and repeating the described procedure. When an array of said static memory cells are present and each is involved in practice of the method, it should be appreciated that a digital memory function, functionally essentially transparent to that enabled by multiple dipole cell providing magnetic media, is enabled.
The writing function can involve application of Pulse Voltage which xe2x80x9cImprintsxe2x80x9d the cell, and said writing can occur in as short a time as 100 nanoseconds in cells sized on the order of 100 nm.
The disclosed invention can then be described as a two terminal bistable memory cell comprising at least two high-spin polarization magnetic material junctions, said junctions being separated from one another by insulator, said two terminal bistable memory cell demonstrating at least two stable hysteretic resistance states; said hysteretic resistance states being set by application of positive or negative D.C. current flow therethrough, wherein the insulator is Cr2O3 and the high-spin polarization magnetic material junctions are formed by contact with thereof with CrO2.
The disclosed invention can generally be described as a two terminal bistable memory cell comprised of at least two high-spin polarization magnetic material junctions, each thereof being formed by ferromagnetic material in contact with the insulator. Further, said insulator material is typically formed to contain trap site defects, and in fact the presence of trap site defects alters the resistance states achievable.
Investigation of fabricated disclosed invention two terminal bistable memory cells have shown that they are characterized in that:
application of a voltage of about a millivolt or less across one CrO2/Cr2O3/CrO2 sequence causes change in zero bias kilo-ohm range resistance of at least ten percent;
application of a magnetic field substantially cancels the difference in observed resistance states effected by sequential application of positive/negative or negative/positive D.C. current flow; and
heating the memory cell to a temperature in excess of the Curie temperature of the magnetic material re-establishes the memory cell capability to be set to two stable hysteretic resistance states by application of positive or negative D.C. current flow therethrough.
A general presently disclosed method of setting and erasing memory comprising the steps of:
a. providing a two terminal bistable memory cell comprising at least two high-spin polarization magnetic material junctions, said junctions being separated from one another by insulator, said two terminal bistable memory cell demonstrating at least two stable hysteretic resistance states; said hysteretic resistance states being set by application of positive or negative D.C. current flow therethrough;
b. by applying a D.C. voltage across said series of at least two high-spin polarization magnetic material comprising junctions, causing a D.C. current to flow therethrough, thereby setting a resistive memory state in said memory cell;
c. monitoring the resistance across said series of at least two of said at least two high-spin polarization magnetic material junctions by application of a resistance determining A.C. or D.C. current flow therethrough;
d. applying a magnetic field to said at least two high-spin polarization magnetic material junctions to erase the memory state set in step b.
Said general method of setting and erasing memory can further comprises the steps of:
e. heating said two terminal bistable memory cell to the Curie temperature of said at least two high-spin polarization magnetic material comprising junctions;
f. by applying a D.C. voltage across said series of at least two high-spin polarization magnetic material comprising junctions, causing a D.C. current to flow therethrough, thereby setting a memory state in said memory cell;
g. monitoring the resistance across said series of at least two of said at least two high-spin polarization magnetic material junctions.
Said general method of setting and erasing memory can further comprise, as an initial step, the heating of said two terminal bistable memory cell to the Curie temperature of said at least two high-spin polarization magnetic material comprising junctions to prepare the system for use as a memory device.
This can be followed by lowering the temperature of the two terminal bistable memory cell to below 250 degrees Kelvin.
A specific presently disclosed method of setting and monitoring memory, comprises the steps of:
a. providing a two terminal bistable memory cell comprising at least two high-spin polarization magnetic material junctions, said junctions being formed between CrO2 and Cr2O3 in at least one sequence of CrO2/Cr2O3/CrO2, said two terminal bistable memory cell demonstrating at least two stable hysteretic resistance states, said hysteretic resistance states being set by application of positive or negative D.C. current flow therethrough;
b. by applying a D.C. voltage across said series of at least two junctions formed between CrO2 and Cr2O3, causing a D.C. current to flow therethrough thereby setting a resistive memory state in said memory cell;
c. monitoring the resistance across said series of at least two of said at least two junctions formed between CrO2 and Cr2O3, by causing an A.C. or D.C. current to flow therethrough.
Said presently disclosed specific method of setting and monitoring memory can further comprise the steps of:
causing a D.C. current to flow therethrough, thereby setting a resistive memory state in said memory cell; and
monitoring the resistance across said series of at least two of said at least two junctions formed between CrO2 and Cr2O3 by flowing an A.C. or D.C. current therethrough;
wherein the D.C. current which sets the resistive memory state which is at least an order of magnitude larger than is the resistance monitoring A.C. or D.C. current.
Said presently disclosed specific method of setting and monitoring memory can further comprise the step of:
d. applying a magnetic field to said at least two high-spin polarization magnetic material junctions to erase the memory state set in step b.
Said presently disclosed specific method of setting and monitoring memory can further comprise the steps of:
e. heating said junctions in said two terminal bistable memory cell to the magnetic material CrO2 Curie temperature of about 400 degrees Centigrade;
f. by applying a D.C. voltage across said series of at least two junctions formed between CrO2 and Cr2O3, causing a D.C. current to flow therethrough thereby setting a resistive memory state in said memory cell;
g. monitoring the resistance across said series of at least two junctions formed between CrO2 and Cr2O3 by flowing an A.C. or D.C. current therethrough.
Said presently disclosed specific method of setting and erasing memory can comprise, as an initial step, the heating of said two terminal bistable memory cell to the Curie temperature of said at least two junctions which comprise CrO2.
It is to be understood that a multiplicity of presently disclosed conventional two terminal bistable memory cells can be fashioned into an array and accessed as are conventional known arrays of two terminal bistable memory cells.
It is also to be understood that while CrO2/Cr2O3/CrO2 is, described herein as comprising two junctions, namely CrO2/Cr2O3 and Cr2O3/CrO2, the CrO2/Cr2O3/CrO2 system is often simply refered to as xe2x80x9ca junctionxe2x80x9d. There is no technical distinction implied by the language adopted in this Disclosure, but said language is adopted only to make clear the Cr2O3 separates two regions of CrO2 which are junctioned to it, from one another.
Finally, it is emphasized that while hysteretic resistance states are set by applying voltage across the terminals of a disclosed invention cell, spin polarized current caused to flow through high-spin polarization magnetic material junctions is what is effective in setting of resistance states in a disclosed inventions cell. Microstructural defect sites, field ionization of electron traps as well as spin-transfer models present explanation for the results achieved. Further, it is noted that where CrO2/Cr2O3/CrO2 systems are involved, the basis of operation is believed to be based in that the Cr2O3 comprises trap site defects, (eg. Cr3+ and/or Cr4+), each said trap site defect being comprised of at least two domains, which xe2x80x9cdomainsxe2x80x9d are caused to be set either parallel or antiparallel to one another depending on the polarity of the applied D.C. hysteretic resistance setting positive or negative polarity D.C. current flow. When the trap site defect domains are caused to be aligned parallel by application of the positive or negative polarity D.C. voltage, one hysteretic resistance state is set, and where the trap site defect domains are caused to be aligned anti-parallel by application of the negative or positive D.C. voltage, the other hysteretic resistance state is set.
For insight, it is noted that stray magnetic field non-volatile memory devices are typically tunnel magneto-resistance systems, while spin transfer non-volatile memory devices are typically comprised of multiple metallic layers. The disclosed invention is a hybrid of the two in that tunnel junctions are formed between non-insulator magnetic material, (eg. CrO2), and trap defect containing insulator, (eg. Cr2O3).
The disclosed invention will be better understood by reference to the Detailed Description Section of this Specification.
It is therefore a general objective and/or purpose of the described invention to disclose two terminal bistable memory cells comprising at least two high-spin polarization magnetic material to insulator junctions, said junctions being separated from one another by said insulator, said two terminal bistable memory cells demonstrating at least two stable hysteretic resistance states; said hysteretic resistance states being set by application of positive or negative D.C. current flow therethrough.
It is a specific objective and/or purpose of the described invention to disclose two terminal bistable memory cells comprising at least two high-spin polarization magnetic material to insulator junctions, said junctions being separated from one another by said insulator, said two terminal bistable memory cells demonstrating at least two stable hysteretic resistance states; said hysteretic resistance states being set by application of positive or negative D.C. current flow therethrough; wherein the insulator is Cr2O3 and the high-spin polarization magnetic material junctions are formed by contact with thereof with CrO2, and in which said insulator are present electron trap site defects.
It is another objective and/or purpose of the described invention to disclose that, while not limiting, junctions between Cr2O3 and CrO2 can be formed by oxidation of chromium or vacuum deposition etc.
Other objectives and/or purposes will become apparent by a reading of the Specification and Claims.